The present invention relates to a device and method for testing a can for leakage and to a system for testing cans for leakage.
Placement of food products into containers during commercial packaging operations poses challenging quality control problems. One such problem arises where the semi-fluid nature of a product causes entrapment of air with the product during packaging. For example, placement of dough in spiral-wound composite containers typically traps air between the product and a closed end of the containers.
It is desirable to remove entrapped air from food containers since entrapped air may cause undesirable characteristics in the food product. As one example, food which is left in contact with entrapped air for excessive periods of time becomes discolored and eventually spoils. A discolored food product is unacceptable to an end user since it is not what the user expected and is unappetizing.
Containers in which semi-fluid food products are packaged may include a vent feature for venting entrapped air following packaging of the food product. For instance, spiral-wound composite containers in which dough products are packaged may include a vent structure at an end of the composite containers. As one example, a metal end of some composite containers is crimped onto an end of a composite wall of the containers such that entrapped air escapes through the crimped joint when elevated pressures exist in the containers.
Elevated pressure sufficient to vent entrapped air through the crimped joint develops for various reasons. Many products, including dough, expand when subject to increased temperatures, thereby pressurizing the container to some extent. Increased pressures also arise due to chemical reactions involving products packaged within containers. Taking dough as an example, leavening agents contained in dough will react to raise pressure in containers when suitable conditions exist.
It is desirable for a commercial packager to have a method and apparatus for testing a suitable number of containers to determine whether the container vent feature is performing satisfactorily. One such apparatus includes a cylindrical test head with one or more seal rings disposed about the periphery of the test head. The test head includes a threaded bore for introducing a pressurized fluid, such as air, for testing the vent feature.
Testing of a container using this apparatus includes placing the open end of the container over the test head such that the test head receives the container. Next, the cylindrical wall of the open end of the container is advanced past the seal ring(s) to seal the container for testing. After the can is positioned on the test head, with the seal ring(s) sealing against the internal cylindrical wall of the container, a positioning bar may be placed over the test head and against the closed end of the container to secure the container on the test head.
A pressurized fluid such as air is introduced through the bore of the test head and into the sealed chamber of the container. The term "sealed chamber", for purposes of this disclosure, means the volume of the tested container which is sealed and enclosed by the test apparatus. After shutting off the flow of pressurized fluid into the sealed chamber, the drop in pressure within the sealed chamber due to leakage from the container is measured over a set time period. The rate of leakage from the sealed chamber is calculated using an equation which includes appropriate pressure drop variables.
Determining leakage rates using this apparatus and method has been found to be unsatisfactory for several reasons. For example, practitioners have discovered that it is necessary to place tape around the container wall proximate the open end of the spiral-wound container. The tape allows the spiral-wound joint of the container to withstand the forces the seal ring(s) exert on the spiral wound joint as the open end and internal wall of the container is forced over and by the seal ring(s). Without the taping, the spiral-wound joint sometimes fractures and the container sometimes splits open due to passage over the seal ring(s), rendering the test futile. Practitioners have also discovered that silicone grease typically must be applied to the spiral-wound joint to reduce friction and assure sealing between the spiral wound joint and the test apparatus as the container is placed onto the test apparatus.
Additionally, forces exerted on the closed end of the container to force the open end of the container by the seal ring(s) may damage the crimped joint of the closed end of the container. Damaged crimped joints may change the leakage characteristics of containers such that leakage rates determined during testing are invalid and misleading.
Another problem arises because the seal created by the seal ring(s) is created as the container is placed onto the test head. If the test apparatus is not vented to atmosphere as the container is forced over the seal ring(s), pressure builds inside the container as the open end of the can advances beyond the seal ring(s). The raised pressure increases the force required to position the container on the test apparatus.
The raised pressure aggravates the force-induced damage to the crimped joint at the closed end of container. The increased pressure may also inadvertently expose the container to a greater pressure than the desired test pressure, thereby damaging the spiral-wound joint and the crimped joint. The damaged joints may cause the container to burst during testing, rendering the test useless. Also, the spiral-wound joint may leak and the crimped joint may have different leakage characteristics, rendering any leak test results invalid.
A further problem is that the described test apparatus and method is highly dependent upon the volume of the system, including the volume of the test apparatus and of the sealed chamber of the container tested. The equation used to calculate the rate of leakage from the container depends upon several variables including the change in pressure in the container during testing, the amount of time during which the pressure change occurred, and the combined volume of the test apparatus and the sealed chamber.
Testing using the described test apparatus is also problematic because of difficulties encountered in controlling and determining the volume of the sealed chamber and the volume of the test apparatus. The volume of the sealed chamber depends upon several variables including the location of the seal rings on the test head, the length and diameter of the container tested, and container dimension changes created in the wall of the container by forcing the container past the seal ring(s). The test apparatus volume includes that volume located between the test apparatus air shut-off valve and the point where the sealed chamber and the bore for introducing pressurized fluid meet.
As noted, measurement of several variables including the following is required before the leakage rate is calculated:
1. Initial test pressure. PA1 2. Final test pressure. PA1 3. Time between measurement of initial and final test pressure. PA1 4. Sealed chamber volume. PA1 5. Test apparatus volume.
Measurement of multiple variables such as these requires excessive time and introduces sources of error. Therefore, it is desirable when testing a container for leakage to reduce the number of variables which must be controlled or determined.
Additionally, the described test apparatus and method are only usable for testing containers within a very narrow range of internal diameters. Factors including the dimensions of the test head and the thickness of the seal ring(s) affect the narrow range. To test containers with diameters outside the narrow range, seal ring(s) with a different thickness and/or a testing apparatus with different dimensions are required. Thus, the described test apparatus and method are not flexible enough to quickly test different containers with more than nominally different internal diameters.